Add JSONPointer to SyntacticError

[dsnet]

Make SyntacticError precise about the position of the error by also recording the JSON pointer.

Other changes:

• Make SyntacticError wrap an error.
• Export ErrDuplicateName as a means to programmatically identify a duplicate object name error.
• Export ErrNonStringName to indicate a syntax error when the expected JSON object name is not a string.
• Always wrap io.ErrUnexpectedEOF in SyntacticError.
• Avoid any dependency injection between jsontext and jsonwire.
• Add convenience methods to jsontext.Pointer.

[johanbrandhorst]

This is super cool - my only comments are that we should add some examples of using the syntactic error to extract information about the error. Maybe we have some and I missed it?

[dsnet]

Examples can come later. This is the first in a sequence to improve error reporting.

After this change, we now prints errors like:

• jsontext: unexpected EOF after offset 13
• jsontext: invalid UTF-8 within "/fizz/buzz" after offset 192
• jsontext: invalid character 'E' within literal false (expecting 'e') after offset 12
• jsontext: duplicate object name "foo" within "/fizz/buzz"

Of particular note, we specially handled ErrDuplicateName in the SyntacticError.Error method for better readability. Without special handling, it would print as jsontext: duplicate object name within "/fizz/buzz/foo", which is technically correct, but fails to highlight "foo" as the offending name.